===================Speculation Control=================== Quite some CPUs have speculation-related misfeatures which are infact vulnerabilities causing data leaks in various forms even acrossprivilege domains. The kernel provides mitigation for such vulnerabilities in variousforms. Some of these mitigations are compile-time configurable and somecan be supplied on the kernel command line. There is also a class of mitigations which are very expensive, but they canbe restricted to a certain set of processes or tasks in controlledenvironments. The mechanism to control these mitigations is via:manpage:`prctl(2)`. There are two prctl options which are related to this:  * PR_GET_SPECULATION_CTRL  * PR_SET_SPECULATION_CTRL PR_GET_SPECULATION_CTRL----------------------- PR_GET_SPECULATION_CTRL returns the state of the speculation misfeaturewhich is selected with arg2 of prctl(2). The return value uses bits 0-3 withthe following meaning: ==== ===================== ===================================================Bit  Define                Description==== ===================== ===================================================0    PR_SPEC_PRCTL         Mitigation can be controlled per task by                           PR_SET_SPECULATION_CTRL.1    PR_SPEC_ENABLE        The speculation feature is enabled, mitigation is                           disabled.2    PR_SPEC_DISABLE       The speculation feature is disabled, mitigation is                           enabled.3    PR_SPEC_FORCE_DISABLE Same as PR_SPEC_DISABLE, but cannot be undone. A                           subsequent prctl(..., PR_SPEC_ENABLE) will fail.==== ===================== =================================================== If all bits are 0 the CPU is not affected by the speculation misfeature. If PR_SPEC_PRCTL is set, then the per-task control of the mitigation isavailable. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculationmisfeature will fail. PR_SET_SPECULATION_CTRL----------------------- PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, whichis selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to handin the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE orPR_SPEC_FORCE_DISABLE. Common error codes------------------======= =================================================================Value   Meaning======= =================================================================EINVAL  The prctl is not implemented by the architecture or unused        prctl(2) arguments are not 0. ENODEV  arg2 is selecting a not supported speculation misfeature.======= ================================================================= PR_SET_SPECULATION_CTRL error codes-----------------------------------======= =================================================================Value   Meaning======= =================================================================0       Success ERANGE  arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor        PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE. ENXIO   Control of the selected speculation misfeature is not possible.        See PR_GET_SPECULATION_CTRL. EPERM   Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller        tried to enable it again.======= ================================================================= Speculation misfeature controls-------------------------------- PR_SPEC_STORE_BYPASS: Speculative Store Bypass   Invocations:   * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0);   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0);   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0);   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0); - PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes                        (Mitigate Spectre V2 style attacks against user processes)   Invocations:   * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0);   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0);   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0);   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);